1. Field of the Invention
The present invention relates to an acoustic wave device, and specifically to a protective film of an acoustic wave device.
2. Description of the Related Art
In general, in semiconductors and acoustic wave devices, such as surface acoustic wave devices, protective films are provided to prevent corrosion of the electrodes and other elements caused by moisture.
For example, for a surface acoustic wave device shown in a sectional view of FIG. 7, a configuration is disclosed in which a substrate 101 mounted on a base plate 103 is covered with a protective film 108. In detail, conductive structures (not shown), such as an IDT electrode, are provided on the substrate 101. The conductive structures on the substrate 101 are sealed with the protective film 108. The substrate 101 is electrically connected to a conductor 104 of the base plate 103 through bumps 105. An insulating layer 106 is disposed between the substrate 101 and the base plate 103, and an enclosing frame 107 is provided in a connection region between the substrate 101 and the base plate 103 using a resin or other suitable material (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-510666).
As a protective film, for example, a silicon nitride film is used. The term “silicon nitride film” generally represents a film having a Si:N composition ratio of 3:4 corresponding to stoichiometric ratio Si3N4 and when the film is represented by SiNX, X=4/3=1.33.
However, the function of preventing the entrance of moisture is deteriorated due to oxidation with moisture and moisture absorption of the protective films.
In particular, in a surface acoustic wave device which causes corrosion of an electrode and a change in material constants with a small amount of moisture, corrosion of an IDT electrode can be suppressed to some extent by a protective film. However, a problem occurs in which SAW energy is also distributed in the protective film, and thus, the sonic speed of the protective film is changed by a change in material constants of the protective film due to moisture, thereby deteriorating the characteristics.